This invention relates to improvements in paper making machine foils which are commonly used in the forming sections of paper making machines, such as of the fourdrinier type wherein water is removed through a wire on which the fiber-water slurry has been deposited. The foil blades are applied to the underside of the wire and have leading side water stripping edges and trailing foiling surfaces, that is surfaces which by angling away from the on-running wire exert negative pressure to draw water through the wire for stripping by the next succeeding foil leading edge.
Numerous and sundry prior foil structures have been proposed to meet problems relating to mounting of the foils, resisting wear at the dewatering tip or on top of the foil and the like.
With increase in machine operating speeds in the constant effort to increase machine productivity, problems relating to the dewatering foils have become more pronounced, and in some instances problems, which at slower speeds have either not been recognized or have been of such little consequence as to be ignored, now have assumed major importance.
By way of example of various attempts to solve one or more problems in dewatering foils, reference is made to the following U.S. Pat. Nos. 3,446,702; 3,732,142; 3,738,911; 4,004,969; 4,134,788 provide a wear surface back of the blade tip but ignore wear of the relatively soft surface of the tip configuration which is easily lost and the blade rendered useless in a comparatively short time.
U.S. Pat. No. 3,778,342 discloses an almost impossible approach from a manufacturing standpoint, although addresses the problem of providing a wear resistant tip but ignores the protection of the upper land portion back of the tip.
U.S. Pat. No. 3,793,140 although providing a wear resistant tip is not usable on a high speed machine because of the arcuate top surface of the tip.
U.S. Pat. No. 3,870,597 provides a removable tip insert but provides no wear resistance for the doctor tip nor wear resistance protection on the rearward land surface.
A salient deficiency with respect to all of the foregoing patents, in addition to shortcomings already mentioned, is that none of the patentees apparently recognized how the wire fabric and the foils react at high speed.
From a study of the wear patterns on existing foil designs, it has been noted that wear takes place in the dewatering or skimming tip and at a point about one-half inch back from the tip. As water rejection occurs below the tip, the phenomenon of stagnation or flow reversal tends to occur as the water drops down from the skimming tip. This causes the forming wire to lift off of the foreward part of the blade tip and then to touch downstream from the tip. Back pressure is somewhat dependent on how much the water stream under the wire is changed in direction. Because of the high angle, generally about 60.degree. between the top of the blade tip and the foreward sloping face of the foil body, all of the above listed patent structures are subject to the stagnation pressure to at least a substantial degree, and thus inefficient at high machine speeds.
Another problem of some magnitude encountered in the prior foil structures is involved in the means for mounting the foil blades. While the patents disclose connections such as dovetail, T-bar, and the like which enable easy slide on and off of the blades with respect to their mountings, the various structures heretofore have presented various machining problems and/or sacrifice in foil body strength.